If you’re using a model or controller from an engine in Rails 2.3.2 you may encounter some crazy errors from time to time. Errors like: A copy of ApplicationController has been removed from the module tree but is still active! Or sometimes the even weirder one: can't dup NilClass

In one situation where you get A copy of ApplicationController has been removed from the module tree but is still active! it could be because you’re using a plugin or engine that provides controllers and inherits from ApplicationController. Some of the classes inherited or included in your engine controllers may fail to get unloaded and cause trouble after the first request to your system. Add unloadable inside your controller classes in your engine.

The can't dup NilClass error really tricked me though. It seemed to be saying something about duping nil class but not really. It was a lie! Well, almost. See, I had a model in my engine like this:

class Account < ActiveRecord::Base
  include Authentication::ByCookieToken
  include Authentication::AccountBuddy
 
  has_many :logins, :dependent => :destroy
 
  attr_accessible :nickname, :email

And I had a module in my app (not in the engine) like this (sort of a reverse-micro-plugin):

module Authentication
  module AccountBuddy
    def self.included(account)
      account.class_eval do
        has_many :characters
      end
    end
  end
end

And when I tried to access account.characters it was all “can’t dup NilClass”. But what it really meant to say was: “I’m returning nil instead of an array containing the characters belonging to this account because I became confused during the loading and unloading of all your crazy ass models. -Love, Rails”

So adding a little unloadable also fixes that.

class Account < ActiveRecord::Base
  unloadable # <= That's the ticket!
  include Authentication::ByCookieToken
  include Authentication::AccountBuddy # reverse-micro-plugin 
 
  has_many :logins, :dependent => :destroy
 
  attr_accessible :nickname, :email

What a load off.

A better way

Courtesy of Paul (from the comments)

unloadable is now deprecated, I think. As far as I am aware, the “right” way to do this is one of the following:

First, you could put something like this in your plugin’s init.rb file:

# This plugin should be reloaded in development mode.
if RAILS_ENV == 'development'
  ActiveSupport::Dependencies.load_once_paths.reject!{|x| x =~ /^#{Regexp.escape(File.dirname(__FILE__))}/}
end

Second, you could put something like this in your application’s environment.rb file:

config.reload_plugins = true if RAILS_ENV == ‘development’

If the above did not solve your problem:

Courtesy of Evan Owen (from the comments)

Another possible cause of this problem is that ActiveRecord also defines self.included. In order to ensure that ActiveRecord gets the call and can finish loading the model, a call to super needs to be added to the end of self.included like so:

def self.included(account)
  account.class_eval do
    has_many :characters
  end
 
  super # fixes the "can’t dup nil" issue
end

This did not work in my specific case, but may be the correct solution if you are suffering from a similar but different problem.